Rechargeable Lithium polymer batteries manufactured from laminates of solid polymer electrolytes and thin film cathodes and thin film lithium metal anodes displays many advantages over conventional lithium-ion batteries using liquid electrolytes. These advantages include lower overall battery weight, high power density, high specific energy, longer service life, and environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated. Lithium metal polymer batteries have become the most promising energy storage device for use in electric or hybrid vehicles and stationary applications such as back-up power systems, energy accumulators for solar panels or wind turbines energy productions and for peak shaving electric consumption in residential and industrial applications.
The production of thin films of lithium having a thickness lower than 100 micrometers (μm) and in the form of wide bands, for example 10 centimeters (cm) or more and in lengths of many hundreds of meters, by means of rapid and reliable processes, faces important technical difficulties which are attributable to the extreme physical and chemical properties of lithium metal such as its chemical reactivity, its malleability, its low mechanical strength, its rapid self-welding by simple contact and its strong adhesion to most solid materials, such as for example other more usual metals like steel, aluminium and their usual alloys.
Presently, cold extrusion is used for the continuous production of lithium or lithium alloy sheets of 150 to 250 μm as described in U.S. Pat. No. 6,854,312 which is herein incorporated by reference taking advantage of the malleability of lithium metal. For lower thicknesses, the films obtained by extrusion are thereafter laminated through a lamination apparatus as described in U.S. Pat. No. 7,513,136 which is also herein incorporated by reference comprising adjustable lamination rollers adapted to control the shape and profile of the lithium or lithium alloy film being laminated at to thicknesses of 100 μm or less which can be used in Lithium polymer batteries.
In the lamination process, a lubricating agent including a specific additive mixed with a solvent as described in U.S. Pat. No. 5,837,401 is used to prevent adhesion of the thin lithium film in contact with the lamination rollers under pressure in order to prevent breakage of the thin lithium film. The particular additive and solvent disclosed in U.S. Pat. No. 5,837,401 which is herein incorporated by reference have the specific property of being chemically compatible with lithium films for use in an electrochemical cell. The lubricating agent described in U.S. Pat. No. 5,837,401 prevents the laminated lithium film from excessively adhering to the lamination rollers and does not react with lithium through oxidation on the surface of the lithium films and therefore does not impair the electrochemical exchanges at the interface of the thin film lithium anode and the polymer electrolyte of the electrochemical cell when the latter is assembled and operated. The main advantage of the chemical formulation of this lubricating agent is that the specific additive may be kept at the surface of the lithium film after lamination and this without harming the good operation of the lithium anode when used as such in an electrochemical cell. In turn, the chemical compatibility of this lubricating agent toward lithium eliminates the step of washing the surfaces of the laminated lithium film prior to assembly and use in an electrochemical cell. Previous lamination lubricant including reactive organic functions, such as organic acids and alcohols used to laminate lithium sheets into lithium films for electrochemical cells had to be removed from the surfaces of the laminated lithium films because they were harmful for a good operation of electrochemical cells.
The preparation of the lubricating agent described in U.S. Pat. No. 5,837,401 is however cumbersome. The composition of the lubricating agent must contain precisely 0.2% by weight of polyoxyethylene distearate mixed with a solvent selected amongst heptane, benzene, toluene, cyclohexane or a mixture of these solvent. The mixture must be stirred for a few hours to properly blend all the components and must be stirred constantly when used as a lubricant agent when laminating lithium sheets into lithium films.
Furthermore, although the lubricant agent remaining on the surface of the lithium film does not hinder the good operation of the lithium anode when is used in an electrochemical cell, it does not contribute to its good operation. As such, remaining lubricant adds to the weight of the electrochemical cell and therefore reduces although marginally the specific energy density of the electrochemical cell produced.
The mixing and blending operation of the lubricating agent described above as well as the direct costs of components of the lubricant agent: the additive (polyoxyethylene distearate) and of the solvents (heptane, benzene, toluene and cyclohexane) increase the overall cost of the electrochemical cells manufactured with thin lithium films.
Thus, there is a need for a new lubricant for laminating lithium sheets into lithium films for electrochemical cells that does not remain on the surface of the lithium films after lamination and that requires less manipulation for its preparation.